Defects inspection is very important in terms of quality control of substrate manufacturing. For example, In the filed of glass manufacture, various types of defects such as scratches, stains, tin pick-ups, chips in terms of surface defects, and bubbles, stones, knots in terms of internal defects, may be formed in glass substrate during the process of manufacturing. Tasks of defect inspection are not only to defects but also to accurately categorize these defects since specifications of quality control are different for various types of defects.
Optical methods are widely applied in defect inspection. In general, there are collimated illumination and diffusive illumination in terms of way of illumination; bright field detection and dark field detection in terms of way of image acquisition; transmission detection and reflection detection in terms of direction of light propagating at surface of inspected object. Different methods need to be applied to characterize different types of defects. For example, collimated illumination is sensitive to deformation or inhomogeneity of refractive index, which can be used to inspect knots or stones. Diffusive illumination is usually used for inspecting core size of defect. Tin pick-up is easy to be inspected under reflection inspection due to high reflectivity. Generally, combination of different methods of inspection is an efficient way to detect and categorize different type of defects.
WO patent application No. 2007/045437 entitled “System and Method for Optically Inspecting Glass Panes” which was published on Apr. 26, 2007 discloses a multi-channel inspection system, where glass panes to be detected passes through three separate inspection modules in series. And each module have respective illumination component, sensing component and image processing component to provide respective inspection of different types of defects. Disadvantages of this system are high cost and large size due to each detection module will have a full set of illumination component, sensing component and image processing component. U.S. patent application No. 2007/0263206 entitled “Apparatus and Method for Characterizing Defects in a Transparent Substrate” which published on Nov. 15, 2007 relates to an inspection system with mixed multiple illumination, where multiple light sources illuminate inspected substrate simultaneously, and the at the same time imaging device captures mixed signals of bright field and dark field. Compared with separated illuminations of each light source, image features of defects under the mixed illumination were reduced because of interference between each illumination. As a result, difficulty of image processing for defects detection and categorization will be increased.
Usually, washing process of substrate is required before inspection of defects since foreign particles such as dust on surface of the substrate may result in misclassification of the particles misclassified as real defects. It will undoubtedly increase fake defects rate of inspection (i.e. the probability of categorizing fake defects as real defects) and consequently increase waste of qualified product. However, a washing machine is typically expensive in terms of cost of the equipment and electric power consumption. On the other hand, washing process sometimes is not feasible or has negative influence on the substrate. One example is inspection of glass ribbon on float line. Another example is glass substrates of flat planes display module, which have a thickness of around 0.7 mm and are likely to break during washing process due to small thickness. A further example is that water mark on the substrate is not acceptable for some applications.
Therefore, there is a need to provide a defect inspection method and defect inspection system which provide high performance to price ratio with compact multi-channel configuration and without interference of multiple illuminations. Beside that, there is further requirement to provide a system and method capable of detecting and categorizing defects of unclean substrate.